Phase 6: Admin portal

Adds is_site_admin-gated site administration: user management (list,
deactivate/reactivate, reset password, promote/demote), room management
(list all rooms including private ones, archive/unarchive, force-transfer
ownership), and an audit log of every admin action.

Backend: User.is_active (deactivation) and Room.is_archived (archive) are
new columns; AdminAuditLog is a new table matching ARCHITECTURE.md's
admin_audit_log design, written to in the same transaction as every
mutating admin action. require_site_admin (dependencies.py) gates all
/api/admin/* routes. get_current_user now rechecks is_active on every
request, so deactivating a user kills their already-open session
immediately, not just future logins. An admin can't deactivate or demote
their own account (the one self-lockout guard included). Archived rooms
drop out of the open-room browse list but stay readable for existing
members.

Frontend: new /admin route (AdminRoute guard, redirects non-admins to
/rooms) with a tabbed Users / Rooms / Audit log / Settings page, plus an
"Admin" link in the account menu for site admins.

Bot/integration management and system settings -- both listed in the
original issue -- are intentionally not here: bot management has nothing
to manage until Phase 7 builds the actual bot data model, and there's no
settings storage or concrete setting to configure yet. Settings has an
empty placeholder tab; bot management is deferred entirely to Phase 7.
Confirmed this scope cut with the repo owner before implementing.

New tests/test_admin.py (14 tests, full suite now 58/58) covers every
admin endpoint's permission gate, the self-action guards, deactivation's
immediate effect on an already-open session, and that every mutating
action produces exactly one audit log row.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/api/admin.ts

@@ -0,0 +1,52 @@
+import { apiFetch } from './client'
+import type { AdminRoom, AdminUser, AuditLogEntry } from '../types'
+
+export function listAdminUsers(): Promise<AdminUser[]> {
+  return apiFetch<AdminUser[]>('/api/admin/users')
+}
+
+export function deactivateUser(userId: string): Promise<AdminUser> {
+  return apiFetch<AdminUser>(`/api/admin/users/${userId}/deactivate`, { method: 'POST' })
+}
+
+export function reactivateUser(userId: string): Promise<AdminUser> {
+  return apiFetch<AdminUser>(`/api/admin/users/${userId}/reactivate`, { method: 'POST' })
+}
+
+export function resetUserPassword(userId: string, newPassword: string): Promise<void> {
+  return apiFetch<void>(`/api/admin/users/${userId}/reset-password`, {
+    method: 'POST',
+    body: JSON.stringify({ new_password: newPassword }),
+  })
+}
+
+export function promoteUser(userId: string): Promise<AdminUser> {
+  return apiFetch<AdminUser>(`/api/admin/users/${userId}/promote`, { method: 'POST' })
+}
+
+export function demoteUser(userId: string): Promise<AdminUser> {
+  return apiFetch<AdminUser>(`/api/admin/users/${userId}/demote`, { method: 'POST' })
+}
+
+export function listAdminRooms(): Promise<AdminRoom[]> {
+  return apiFetch<AdminRoom[]>('/api/admin/rooms')
+}
+
+export function archiveRoom(roomId: string): Promise<AdminRoom> {
+  return apiFetch<AdminRoom>(`/api/admin/rooms/${roomId}/archive`, { method: 'POST' })
+}
+
+export function unarchiveRoom(roomId: string): Promise<AdminRoom> {
+  return apiFetch<AdminRoom>(`/api/admin/rooms/${roomId}/unarchive`, { method: 'POST' })
+}
+
+export function transferOwnershipAdmin(roomId: string, newOwnerId: string): Promise<AdminRoom> {
+  return apiFetch<AdminRoom>(`/api/admin/rooms/${roomId}/transfer-ownership`, {
+    method: 'POST',
+    body: JSON.stringify({ new_owner_id: newOwnerId }),
+  })
+}
+
+export function listAuditLog(limit = 50, offset = 0): Promise<AuditLogEntry[]> {
+  return apiFetch<AuditLogEntry[]>(`/api/admin/audit-log?limit=${limit}&offset=${offset}`)
+}